Dredging arrangement comprising a biasing device

ABSTRACT

The invention relates to a dredging arrangement for dredging material from an underwater bottom ( 6 ). The dredging arrangement comprises a drag head body ( 11 ) and a visor ( 12 ), the visor ( 12 ) being connected to the drag head body ( 11 ) and is moveable with respect to the drag head body ( 11 ) over a predetermined range. The biasing device ( 60 ) is provided in between the drag head body ( 11 ) and the visor ( 12 ) exerting a biasing force on the visor ( 12 ) over a biasing portion of the predetermined range, the biasing portion being at least 25% of the range.

TECHNICAL FIELD

The invention relates to a dredging arrangement for dredging materialfrom an underwater bottom. The invention further relates to a dredgingvessel comprising such a dredging arrangement.

The invention also relates to a method for dredging material from anunderwater bottom using such a dredging vessel.

BACKGROUND

Dredging is often done by dragging a drag head over an underwater bottomby a dredging vessel, such as a trailing suction hopper dredger vessel.The drag head is connected to the vessel by means of a suction pipe. Thedrag head is lowered to the underwater bottom by one or more hoistingwires. One or more pumps are provided to suck bottom material from theunderwater bottom via the drag head, the suction pipe into a hopper ofthe dredging vessel.

Different dredging tools are known, for instance from U.S. Pat. No.4,249,324, EP1786982 en U.S. Pat. No. 7,895,775.

Drag heads are known comprising a visor at their trailing end. The visoris rotatable about an axis rotation which is, in use, substantiallyperpendicular to a dredging direction and substantially horizontal orparallel to the underwater bottom being dredged. The visor may bemounted such that it can rotate freely about its axis of rotation.

U.S. Pat. No. 1,840,606 shows a casing and a drag (visor) which isrotatable about an axis of rotation which is substantially perpendicularto a dredging direction and substantially horizontal. On top of the dragare two lugs with slots, wherein lower pins of links are slideablyengaged. The other end of the link is connected to the casing via springshock absorber arrangements. The drag head can rotate freely until thelower pins of the links reach one end or the other of the slots. Thespring shock absorber arrangement is provided to absorb the shock whenthe lower pins reach an end of the slots. In U.S. Pat. No. 1,840,606 thevisor moves freely within its moving range.

A disadvantage of freely moving visors is that the visor will not alwaysfollow the contour of the underwater bottom (trenches, dunes) as it isonly held against the underwater bottom by its weight. Also, such a draghead will give a limited penetration depth of the visor (resulting inless production), due to the limited own weight of the visor. Placingadditional weights on the visor is usually not a cost effective solutionas it requires more hoisting power when lifting the drag head out of thewater. Alternatives are known to overcome this problem.

For instance, it is known to fix the visor in a desired rotationalposition. This solution provides suboptimal results in case of changingdredging conditions, such as changing dredging depths as a result ofswell, tidal motions, changing draught of the vessel when being loadedor unloaded, depth of the underwater bottom etc. and changing soilcharacteristics (e.g. hard packed or loosely packed etc.). Thissolutions also can result in lift of the heel of the drag head (leadingpart) from the soil when positioned to deep (resulting in lessproduction). Also a fixed visor will not be able to cope with obstaclesand may get damaged.

Another known solution is to control the rotational position of thevisor using hydraulic actuators. For example, GB2128663 uses a hydraulicactuator to control the rotational position of the visor. U.S. Pat. No.4,123,859 and U.S. Pat. No. 4,150,502 each use hydraulic actuators tocontrol the position of the cutting tool or chisel. However, this is anexpensive and complex solution, which requires control effort. Hydraulicsystems are prone to leakage and malfunction.

SUMMARY

It is an object to provide an improved dredging arrangement.

Therefore, according to an aspect, there is provided a dredgingarrangement for dredging material from an underwater bottom, thedredging arrangement comprises a drag head body and a visor, the visorbeing connected to the drag head body and is moveable with respect tothe drag head body over a predetermined range, wherein a biasing deviceis provided in between the drag head body and the visor exerting abiasing force on the visor over a biasing portion of the predeterminedrange, the biasing portion being at least 25% of the range.

The predetermined range may be adjustable and may be defined bymechanical stops being provided limiting the movement of the moveableparts.

The force is applied such that the visor is pushed towards theunderwater bottom. The visor may comprise a plurality of teeth mountedon a row substantially parallel to the axis of rotation. The forceexerted by the biasing device pushes these teeth into the underwaterbottom.

This visor can move freely over the remaining portion of the rotationalrange, but may also be controlled over the remaining portion of therotational range, for instance by hydraulics or the like.

By using biasing device, a force is applied to the visor resulting inmore penetration depth, while maintaining compliance to give way toheavy objects.

According to an embodiment the biasing portion is at least 40% of therotational range.

According to an embodiment the biasing portion is at least 60% of therotational range.

According to an embodiment the biasing portion is 80% or 100% of therotational range.

A larger biasing range makes dredging easier, as less control isnecessary during dredging to take into account changing dredgingconditions, e.g. dredging depths.

According to an embodiment the visor is rotatable with respect to thedrag head body about a rotational axis, the predetermined biasing rangebeing a predetermined rotational range, the rotational axis, in use,being substantially parallel to the underwater bottom and perpendicularto a dredging direction, and the predetermined biasing range being apredetermined rotational biasing range. The visor may be connected to atrailing side of the drag head body.

The predetermined rotational range may for instance be set by twomechanical stops limiting the freedom of rotational movement of thevisor, for instance to a range of 20°, 30°, 40°, or 50°.

Alternatively, visors may be used which are moveable in another manner,such being sliceable, translational or are arranged to perform acombination of a rotational and translational movement.

The range may in this case for instance be 50°, wherein the biasingportion of the range is 20° (40%).

According to an embodiment the biasing device exerts a force on thevisor such that the visor is pushed into a downward direction. In use,the visor is pushed towards the underwater bottom.

In case of a rotational visor, the visor is pushed to rotate such thatthe visor effectively moves downwards.

The biasing device is with one end connected to the visor and withanother end connected to another part of the dredging arrangement suchas the drag head body or lower end of the suction pipe such that thebiasing device can apply a force onto the visor. The biasing forcepushes the visor in a (rotational) downward direction.

The biasing device is preferably mounted on top of the dredgingarrangement.

According to an embodiment the biasing portion of the range is an upperportion of the range and the visor moves freely in a remaining lowerportion of the range.

The biasing force is preferably applied over the upper portion of the(rotational) range. The lower portion of the (rotational) range willtypically be used when the dredging arrangement is hoisted or positionedon a rest position on deck of vessel. The lower portion of the(rotational) range will also be used when the dredging arrangement meetsa dip in the underwater bottom.

According to an embodiment the biasing device is adjustable to set thebiasing force being exerted.

By adjusting the biasing force, the behavior of the visor can beadjusted. For instance, in case the biasing device is set to exert arelatively high preloading force, the visor will be pushed into theunderwater bottom relatively deeply, resulting in a relatively largedredging depth. The visor will in that case be relatively stiff when anobstacle is met, such as a rock.

It will be understood that depending on the biasing device used, theactual biasing force may vary over the biasing portion of the(rotational) range. However, by adjusting the biasing force, the biasingforce can be increased or decreased over the entire biasing portion.

According to an embodiment the biasing device comprises spring devices,the spring device being loaded in the biasing portion of the range suchthat the spring device exerts a biasing force on the visor.

A spring device is an advantageous device of providing a biasing device.Also, spring devices have the advantage that they can deal with shocksin a reliable manner, for instance when the drag head is dragged over anobstacle, such as a stone or the like.

Also, spring devices may be employed wherein the biasing force increasesover the biasing portion of the (rotational) range towards the upperposition of the visor.

Spring devices are suitable for creating a predetermined forcecharacteristic along the (rotational) range.

Also, spring devices have the advantages that no external power supplyis needed.

According to an embodiment the spring device comprises one or moretorsion springs.

According to an embodiment the spring device comprises one or more coilsprings.

The coil springs may be orientated such that their longitudinaldirection is parallel to the dredging direction or at an upward anglewith respect to the dredging direction. In case of a rotational visor,the longitudinal direction may be perpendicular to the axis or rotation.The coil spring can be at an angle with respect to the horizontaldirection.

The coil springs may be compression springs, i.e. when unloaded,neighboring coils do not touch each other. The coil springs areconstructed and mounted such that they are compressed over at least theabove defined (rotational) biasing portion of the range.

Of course, other suitable spring devices may be used, such as rubberdevices.

According to an embodiment the biasing device is mounted between thevisor and the drag head body or lower end of a suction pipe.

The suction pipe may be connected to the drag head body, or may beintegrally formed with the drag head body.

The preloading device may be connected to the drag head body or lowerend of the suction pipe in any suitable manner. The connection may be adirect connection or may be established by means of a connection member.The connection may be a rotational connection allowing rotationalmovement of the biasing device about an axis of rotation parallel to therotational axis of the visor with respect to the drag head body or lowerend of the suction pipe.

The biasing device may be connected to the visor directly. The biasingdevice may be connected to the visor by means of a rotational connectionallowing rotational movement of the biasing device about an axis ofrotation parallel to the rotational axis of the visor with respect tothe visor.

Alternatively or additionally, the connection allows sliding or lateralmovement.

According to an embodiment the biasing device is connected to thedredging arrangement by means of a force transmitting construction.

The force transmitting construction comprises two or more rotatablyconnected beams which transfer the force from the preloading means tothe visor. Alternatively or additionally, the force transmittingconstructions allows sliding or lateral movement.

According to an embodiment the force transmitting construction isadjustable to set the moment of force exerted on the visor by thebiasing device.

According to an embodiment, the force transmitting construction isadjustable to set the predetermined range.

According to an embodiment, the moveable part is a visor and the biasingdevice is arranged to exert a biasing force resulting in a moment offorce acting on the visor which is, averaged over the biasing portion atleast 50% of the moment of force acting on the visor as a consequence ofgravity acting on the visor.

Preferably, the moment of force acting on the visor which is, averagedover the biasing portion at least 75% or at least 100% of the moment offorce acting on the visor as a consequence of gravity acting on thevisor.

According to an aspect there is provided a dredging vessel comprising adredging arrangement for dredging material from an underwater bottomaccording to any one of the preceding claims and a suction pipeconnecting the dredging arrangement to the dredging vessel.

According to an aspect there is provided a method for dredging materialfrom an underwater bottom using a dredging vessel comprising a dredgingarrangement, the dredging arrangement comprising a drag head body and avisor, the visor being moveable with respect to the drag head body overa predetermined range,

wherein the method comprises

-   -   adjusting the predetermined range through the use of mechanical        stops,    -   lowering the dredging arrangement to an underwater position with        the drag head positioned on the underwater bottom,    -   dredging by dragging the drag head over the underwater bottom in        a dredging direction by means of a dredging vessel,

the dredging arrangement comprising a biasing device provided in betweenthe drag head body and the visor exerting a biasing force on the visorover a biasing portion of the predetermined range, wherein the methodcomprises,

-   -   adjusting the biasing device to set the biasing force being        exerted.

As described above, the visor may be rotatable with respect to the draghead body, may be provided at the trailing end and the predeterminedrange may be a predetermined rotational range. Adjustment is preferablydone prior to lowering the dredging arrangement.

According to an embodiment the biasing device is connected to thedredging arrangement by means of an adjustable force transmittingconstruction, and the method comprises,

-   -   adjusting the force transmitting construction to set the moment        of force exerted on the visor by the biasing device.

The force characteristic (force as a function of the position of thevisor within the range) can be adjusted prior to lowering the dredgingarrangement.

Adjustment is preferably done prior to lowering the dredgingarrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying schematic drawings in which correspondingreference symbols indicate corresponding parts, and in which:

FIG. 1 schematically shows completed dredging vessel and a dredgingarrangement,

FIGS. 2 a-2 c schematically show a perspective, top and side view of adredging arrangement according to an embodiment,

FIG. 3 schematically shows a side view of a dredging arrangementaccording to an embodiment,

FIGS. 4 a-4 b schematically show graphs describing the embodiments.

The figures are only meant for illustrative purposes, and do not serveas restriction of the scope or the protection as laid down by theclaims.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a dredging vessel 1 according to anembodiment. The dredging vessel 1 comprises a drag head 10 which isattached to the dredging vessel 1 via a suction pipe 2. The suction pipe2 comprises a hinge point half way the suction pipe 2.

The drag head 10 and the suction pipe 2 are shown in a lowered positionwith the drag head 10 resting on an underwater bottom 6. In use, thedredging vessel 1 sails in a dredging direction DD and drags the draghead 10 over the underwater bottom 6.

The drag head 10 and the suction pipe 2 are connected to a wire 4, thewire 4 being controlled by wire controller 5. Half way the suction pipe2, the suction pipe 2 may be connected to a further wire 4′ controlledby further wire controller 5′. The further wire controller and wirecontroller are mainly used for lifting and lowering the suction pipe 2.

The wire controller 5 comprises a first wire control device, in thiscase formed by a controllable winch 51 and a second wire control device.

The wire controller 5 may further comprise a control unit 55 or the liketo control the first, second and further wire control devices. Thecontrol unit 55 may be a standalone control unit or may be arranged tocooperate with other remote control units. The control unit 55 may be acomputer. The control unit 55 may also be arranged to receiveinstructions from an operator via a user interface.

The second wire control device may be a heave compensator, in this caseformed by a number of pulleys 52, at least one of the pulleys 52 beingmoveable in a direction perpendicular to the rotational axis of thepulley 52 by an actuator 53. In FIG. 1, the actuator 53 is an hydraulicactuator comprising a cylinder and a piston which can move up and downthe cylinder. The moveable pulley 52 is connected to the cylinder. Theactuator 53 is also under control of the control unit 55.

The drag head 10 comprises two main parts: the drag head body 11 and thevisor 12. The drag head body 11 is on one side connected to the suctionpipe 2 via appropriate connection means 13. The drag head body 11 andthe (lower end of the) suction pipe 2 may also be formed as one piece.

The drag head body 11 may have any suitable shape and its main purposeis to form a chamber in which an underpressure can be created to vacuumup dredging material from the underwater bottom 6. A pump 21 may beprovided to create the required underpressure. The pump 21 may bepositioned on board of the dredging vessel 1 (as shown in FIG. 1) and/orat or nearby the drag head 10. The drag head 10 may comprise jet nozzles22 to create a water jet to loosen the underwater bottom 6. A jet pump23 and a jet pipe lane 24 may be present.

The visor 12 is a moveable part of the drag head 10.

The visor 12 is moveable with respect to the drag head body 11 to changethe dredging characteristics of the drag head 10. In the embodimentshown, the visor is provided on the trailing end of the drag head 10 andis rotatable about a rotational axis RA, which runs perpendicular to theplane of the drawing of FIG. 1, i.e. parallel to the underwater bottom 6and substantially perpendicular to a dredging direction DD. Rotating thevisor 12 to a lower position (counter clockwise in FIG. 1), results indeeper dredging and a higher sand-water ratio.

The drag head 10 may further comprise a valve 14. As shown in FIG. 1,the valve 14 is attached to the visor 12 and is arranged to open andclose an opening 18 in the visor 12. Alternatively, the valve 14 may beattached to the drag head body 11 and be arranged to open and close anopening 15 in the drag head body 11.

In FIG. 1 the drag head 10 is only shown schematically. Details of thedrag head 10 in accordance with the embodiments of the invention areshown in FIGS. 2 a-2 c and in FIG. 3.

FIGS. 2 a-2 c show the dredging arrangement according to an embodimentin more detail.

FIG. 2 a schematically shows a perspective view of a dredgingarrangement for dredging material from an underwater bottom 6. Thedredging arrangement comprises the drag head body 11 and the visor 12connected to the drag head body 11 in a moveable manner. As shown inFIG. 2 a, the visor is rotatable about a rotational axis RA, butalternative relative movements of the visor 12 and the drag head body 11are also possible, such as a translational movement, possibly incombination with a rotational movement.

The visor 12 is moveable along a predefined path and has a predefinedfreedom of movement, indicated with the term range. According to theembodiment shown in FIGS. 2 a- 2 c and 3 the visor is rotatable withrespect to the drag head body 11 over a rotational range, whichtypically is 50°.

A biasing device 60 is provided on top of the dredging arrangementconnected to the visor 12 and the drag head body 11 arranged to generateand apply a biasing force between the drag head body 11 and the visor12. The biasing force pushes the visor 12 in a rotational downwarddirection, into the underwater bottom 6.

The biasing force is applied over a predetermined portion of the range,for instance over the upper 25% of the range. In the example mentionedabove, in which the rotational range typically is 50°, this results in abiasing force being applied to the visor 12 over the upper 12.5°. Ofcourse, the biasing portion may be any suitable percentage, such as 40%,60%, 80, or 100%. In the remainder of the range (except in case of100%), the visor 12 is not biased.

Different biasing devices 60 may be used, such as torsion spring 61 orcoil springs 63.

A force transmitting construction 62 may be provided to mount thebiasing device, such as the torsion spring(s) 61 or coil spring(s) 63,to the dredging arrangement and ensure that the biasing force is appliedto the visor 12. This will be explained in more detail below.

FIG. 2 a shows torsion springs 61 on top of the dredging arrangement ina direction perpendicular to the dredging direction DD and substantiallyparallel to the underwater bottom 6 (in use). A lever 621, part of theforce transmitting construction 62 is mounted to the torsion springs 61perpendicular to the torsion springs 61 towards a trailing end of thedredging arrangement. The lever 621 is rotatable about longitudinal axis625.

The force transmitting construction 62 further comprises a connectionrod 622, which is rotatable connected to a distal end of the first lever621 forming axis of rotation 627.

As shown in FIG. 2 a, a connector is provided as part of the forcetransmitting construction 62 on top of the visor 12, in this case formedby two lugs 623. The lugs 623 are rotatable connected to the connectionrod 622 forming axis of rotation 624.

The connection rod 622 and the lever 621 are connected by means of arotatable and slideable connection. The connection rod comprises a slotextending over a top portion of the connection rod 622 to which thelever 621 is slideably connected, for instance by means of a bolt.

As more clearly indicated in FIG. 2 c, schematically depicting a sideview of the dredging arrangement, the lever 621 has an arm L1, and theconnection rod 622 has an arm L2.

The force transmitting construction 62 is adjustable to set the momentof force exerted on the visor 12 by the biasing device 60. Adjustmentsmay be made to change the location where the connection rod 622 isattached to the connector in this case formed by the two lugs 623.

Adjusting the force transmitting construction also allows to set thepredetermined range. Stops may be provided to limit the movement of thevisor, for instance, stops may be provided to limit movements of lever621, thereby setting a maximum range for the visor 12. The stops may beprovided to prevent overload of the springs 61, 63.

For instance, if axis of rotation 624 is in the upper opening providedby lugs 623 (as shown in the figures), the range may be 20°, in whichthe visor 12 is biased (biasing portion of 100%).

If axis of rotation 624 is in the second highest opening provided bylugs 623, the range may be 30°, the upper 20° of which may be biased(biasing portion of 66, 6%).

If axis of rotation 624 is in the second lowest opening provided by lugs623, the range may be 40°, the upper 20° of which may be biased (biasingportion of 50%).

If axis of rotation 624 is in the lowest opening provided by lugs 623,the range may be 50°, the upper 20° of which may be biased (biasingportion of 40%).

Spring devices may be used, such as a torsion spring 61, which generatean increasing force when being deformed. The same applies for a coilspring (an example of which is provided with reference to FIG. 3).

The biasing force is therefore not constant over the portion of therange. This may be at least partially overcome by positioning axes ofrotation 624 and 625 with respect to axis of rotation RA of the visor 12and by choosing length L1 and L2 carefully, i.e. such that rotation oflever 621 about axis of rotation 625 as a function of the rotation ofthe visor around axis of rotation RA is such that it cancels out thebehavior of the spring. The typical behavior of a torsions spring isgiven by a linear relation between angle of rotation and torque (seeFIG. 4 a). In order to achieve a constant force being exerted on thevisor 12, a preload can be applied to the torsion springs 61. The ratioD of the change in angle of the torsion spring as a function of thechange in visor angle, D=(d angle_(spring)/d angle_(visor)), ispreferably such that it cancels out the spring behavior. An approximatedexample can be seen in graph 4 b, showing ratio D along the vertical andthe angle of the visor 12 along the horizontal. Point C is determined bythe preload and the desired force. Slope a is determined by the springstiffness and the applied preload.

Effectively there is a gear ratio between the torsion spring 61 and thevisor 12 that changes when the force transmitting device 62 is adjusted.In case of torsion springs the moment of force on the spring times gearratio is the moment of force on visor.

As can be seen from FIGS. 2 a-2 c, the biasing force is applied in theupper portion of the range in which the visor can move. In the lowerportion of the range, the visor 12 can move without being influenced bythe biasing device 60 as lever 621 can slide freely through the slot ofconnection rod 622.

FIG. 3 shows an alternative embodiment, wherein the torsion springs 61are no longer present and instead a linear spring 63 is provided, suchas a coil spring 63, attached to a protrusion 631. The torsion springs61 are replaced with a rod which is rotatable about its longitudinalbody axis 625. The protrusion 631 may be formed as part of lever 621.

The coil spring 63 is mounted between the protrusion and the drag headbody 11 or the lower end of the suction pipe 2.

It will be understood that the dredging arrangement shown in FIG. 2 a-2c and in FIG. 3 can be used in combination with the dredging vessel 1shown in FIG. 1.

The use of the embodiment will now be described in more detail. Theembodiments described can be used in a method for dredging material fromthe underwater bottom. Such a method may involve use of a dredgingvessel 1 as described with reference to FIG. 1 and dredging arrangementsdescribed with reference to FIGS. 2 a-3.

Dredging may be started by lowering the dredging arrangement to theunderwater position until the drag head 10 is positioned on theunderwater bottom 6. Lowering may be done using the wire controller 5described above.

Next, dredging may be done by dragging the drag head 10 over theunderwater bottom 6 in a dredging direction DD by sailing the dredgingvessel in the dredging direction.

The biasing device 60 may be set to set the biasing force being exerted.For instance, the torsion springs or the linear spring may be adjustedto set the biasing force. Torsion springs 61 may for instance be set byrotating the torsion springs 61 with respect to the dredgingarrangement.

However, adjusting the biasing device 60 may also comprise replacing thesprings with alternative springs, having a different forcecharacteristic or changing the amount of springs.

Furthermore, the force transmitting construction may be adjusted to setthe moment of force exerted on the visor 12 by the biasing device 60 maybe set. For instance, the location where the connection rod 622 isattached to the connector, e.g. the two lugs 623, may be adjusted.

The descriptions above are intended to be illustrative, not limiting. Itwill be apparent to the person skilled in the art that alternative andequivalent embodiments of the invention can be conceived and reduced topractice, without departing from the scope of the claims set out below.

1. A dredging arrangement for dredging material from an underwaterbottom, the dredging arrangement comprising a drag head body and avisor, the visor being connected to the drag head body and beingmoveable with respect to the drag head body over a predetermined range,wherein a biasing device is provided in between the drag head body andthe visor exerting a biasing force on the visor over a biasing portionof the predetermined range, the biasing portion being at least 25% ofthe range, wherein the predetermined range is adjustable and defined bymechanical stops.
 2. The dredging arrangement according to claim 1,wherein the biasing portion is at least 40% of the range.
 3. Thedredging arrangement according to claim 1, wherein the biasing portionis at least 60% of the range.
 4. The dredging arrangement according toclaim 1, wherein the biasing portion is 80% or 100% of the range.
 5. Thedredging arrangement according to claim 1, wherein the visor isrotatable with respect to the drag head body about a rotational axis,the predetermined range being a predetermined rotational range, therotational axis, in use, being substantially parallel to the underwaterbottom and perpendicular to a dredging direction.
 6. The dredgingarrangement according to claim 1, wherein the biasing device exerts aforce on the visor such that the visor is pushed into a downwarddirection.
 7. The dredging arrangement according to claim 1, wherein thebiasing portion of the range is an upper portion of the predeterminedrange and the visor moves freely in a remaining lower portion of thepredetermined range.
 8. The dredging arrangement according to claim 1,wherein the biasing device is adjustable to set the biasing force beingexerted.
 9. The dredging arrangement according to claim 1, wherein thebiasing device comprises a spring device, the spring device being loadedin the biasing portion of the range such that the spring device exertsthe biasing force on the visor.
 10. The dredging arrangement accordingto claim 9, wherein the spring device comprises one or more torsionsprings.
 11. The dredging arrangement according to claim 9, wherein thespring device comprises one or more coil springs.
 12. The dredgingarrangement according to claim 1, wherein the biasing device is mountedbetween the visor and the drag head body or a lower end of a suctionpipe.
 13. The dredging arrangement according to claim 1, wherein thebiasing device is connected to the dredging arrangement via a forcetransmitting construction.
 14. The dredging arrangement according toclaim 13, wherein the force transmitting construction is adjustable toset a moment of force exerted on the visor by the biasing device. 15.The dredging arrangement according to claim 13, wherein the forcetransmitting construction is adjustable to set the predetermined range.16. The dredging arrangement according to claim 1, wherein the biasingdevice is arranged to exert the biasing force resulting in a moment offorce acting on the visor which is, averaged over the biasing portion atleast 50% of a gravitational moment of force acting on the visor as aconsequence of gravity acting on the visor.
 17. A dredging vesselcomprising a dredging arrangement for dredging material from anunderwater bottom according to claim 1, and a suction pipe connectingthe dredging arrangement to the dredging vessel.
 18. A method fordredging material from an underwater bottom using a dredging vesselcomprising a dredging arrangement, the dredging arrangement comprising adrag head body and a visor, the visor being moveable with respect to thedrag head body (11) over a predetermined range, wherein the methodcomprises adjusting the predetermined range through the use ofmechanical stops; lowering the dredging arrangement to an underwaterposition with the drag head positioned on the underwater bottom,dredging by dragging the drag head over the underwater bottom in adredging direction by means of a dredging vessel, and wherein thedredging arrangement comprises a biasing device provided in between thedrag head body and the visor exerting a biasing force on the visor overa biasing portion of the predetermined range, wherein the methodcomprises adjusting the biasing device to set the biasing force beingexerted.
 19. The method according to claim 18, wherein the biasingdevice is connected to the dredging arrangement via an adjustable forcetransmitting construction, and the method comprises, adjusting the forcetransmitting construction to set a moment of force exerted on the visorby the biasing device.
 20. (canceled)